Technical Field
The present disclosure relates to a piezoelectric transducer for an energy-harvesting system and to a method for harvesting energy through a piezoelectric transducer.
Description of the Related Art
The disclosure is particularly suited to the production of piezoelectric microtransducers that may be used in miniaturized energy-harvesting systems capable of supplying, among others, electronic components and/or devices, such as low-consumption sensors and actuators, frequently used in portable electronic devices, such as cellphones, tablet computers, portable computers (laptops), video cameras, photographic cameras, consoles for videogames, and so forth.
As is known, systems for collecting energy from environmental-energy sources (also referred to as energy harvesting or energy scavenging systems) have aroused and continue to arouse considerable interest in a wide range of fields of technology. Typically, energy-harvesting systems are designed to harvest (or scavenge), store, and transfer energy generated by mechanical sources to a generic load of an electrical type. In this way, the electrical load does not use batteries or other supply systems that are frequently cumbersome, have low resistance to mechanical stresses and entail maintenance costs for replacement operations. Furthermore, systems for harvesting environmental energy are of considerable interest for devices that are in any case provided with battery supply systems, which, however, have a rather limited autonomy. This is the case, for example, of many portable electronic devices that are increasingly becoming widely used, such as cellphones, tablets, portable computers (laptops), video cameras, photographic cameras, consoles for videogames, etc. Systems for harvesting environmental energy may be used for supplying components or devices in order to reduce the energy absorbed from the battery and, in practice, increase the autonomy.
Environmental energy may be harvested from several available sources and converted into electrical energy by appropriate transducers. For instance, available energy sources may be mechanical or acoustic vibrations or, more in general, forces or pressures, chemical-energy sources, electromagnetic fields, environmental light, thermal-energy sources.
For harvesting and conversion piezoelectric transducers may, among others, be used.
Piezoelectric transducers are in general based upon a microstructure comprising a supporting body, connected to which are piezoelectric cantilever elements, having at least one portion made of piezoelectric material. The free ends of the piezoelectric cantilever elements, to which additional masses can be connected, oscillate elastically in response to movements of the supporting body or to vibrations transmitted thereto. As a result of the movements of bending and extension during the oscillations, the piezoelectric material produces a charge that can be harvested and stored in a storage element.
In miniaturized transducers, however, the use of just the piezoelectric cantilever elements and the additional masses does not enable adequate levels of efficiency to be achieved. In practice, the conversion of kinetic energy is not satisfactory because the natural frequency of the system formed by the piezoelectric cantilever element and by the additional mass is too different from the typical environmental frequencies that can be transduced.
To improve the efficiency of piezoelectric transducers, it has been proposed to use a movable mass separate from the piezoelectric cantilever elements and magnets that enable temporary coupling of the movable mass and piezoelectric cantilever elements. The magnets are arranged in part on the movable mass and in part on the piezoelectric cantilever elements and are oriented so as to exert attractive forces. The movable mass is constrained to the supporting body so as to be able to come into contact with the piezoelectric cantilever elements and enable coupling of the magnets. The piezoelectric cantilever elements are drawn along in motion by the movable mass and undergo deformation until the elastic return force exceeds the magnetic force. At this point, the magnets separate, and the action of the magnetic force on the piezoelectric cantilever elements ceases almost instantaneously as the movable mass moves away, allowing the elastic force alone to act. In practice, this is equivalent to applying a force pulse on the piezoelectric cantilever elements, which are hence stimulated over a wide frequency band, which also includes the resonance frequency.
Albeit far better from the efficiency standpoint, the devices described present, however, some limits in terms of reliability. In fact, each oscillation of the movable mass causes impact between the magnets of the movable mass itself and the magnets of the piezoelectric cantilever elements. Even though the frequency of oscillation of the movable mass is low (generally less than about 10 Hz), in the long run repetition of the impact may cause damage to the microstructure. In particular, microcracks may be formed, which rapidly propagate until they render the transducer unserviceable.